Plants medium-sized, green, yellowish or brownish, never red; branch and shoot apices not pencil-like; cells of stem epidermis not widened; pseudoparaphyllia semi-orbicular to much wider than long, sometimes with irregular apex; axillary hairs with 1--2-celled distal portion, hyaline when young. Stem leaves slightly falcate or ± straight, from ovate or ovate-triangular base gradually narrowed to rather broadly acuminate and often hooked apex (apex incurved over leaf), slightly concave; margins denticulate below, entire or sparsely denticulate above; costa ending 70--85% distally in leaf; alar cells in distinctly delimited, transversely triangular group that ± reaches costa, longly and broadly decurrent, supra-alar cells weakly differentiated. Sexual condition dioicous.

Mineral- and somewhat nutrient-rich fens and on shores, occasionally submerged in lakes; 0--1560 m; Greenland, Alta., B.C., Man., Nfld., N.W.T., Nun., Ont., Que., Yukon; Alaska, Maine; n Eurasia; incorrectly reported from Australia.

Warnstorfia tundrae differs from all other members of the genus by its longly and more or less broadly decurrent stem leaves. Additional features that aid in separating this species from other Warnstorfia species with leaves that are gradually narrowed to the leaf apex are the rather weakly falcate stem leaves and the total lack of red colors.